Reflective colour filter and display device including the same

ABSTRACT

A reflective filter structure ( 30 ) adapted to reproduce colour by reflection, which filter structure comprises a first domain ( 31 ) reflecting visible red colour at zero angle of incidence, as well as a second domain ( 34 ) reflecting invisible infrared colour at zero angle of incidence. Furthermore, the second domain is adapted to reflect visible red colour at a predetermined angle of incidence greater than zero. The invention also relates to a display device based on the reflective filter structure.

The present invention relates to reflective colour filters, and moreparticularly to cholesteric filters for use in connection with displaysdevices. The invention also relates to a display device utilising suchfilters.

Cholesteric materials, also known as chiral nematic materials, can beused as reflective colour filters. One advantage of such filters is thatthey do not absorb any light, in contrast to conventional absorbingcolour filters. Thus, a higher filtering efficiency may be achieved whenusing cholesteric filters rather than conventional, absorbing colourfilters. Furthermore, manufacturing of cholesteric colour filters isless complicated than manufacturing of absorbing colour filters, in therespect that fewer processing steps are typically needed. However, thecolour reflected from cholesteric colour filters is strongly dependenton the viewing angle (i.e. the angle of incidence on the filter). Theangular acceptance bandwidth of display devices of the reflective typebased on cholesteric liquid crystals is typically around ±20°, at most.

A layer of cholesteric material ordered in a planar state (principalaxis perpendicular to the surface of the filter layer) acts as aninterference film that reflects light of a wavelength matching the pitchof the cholesteric structure. Light not meeting the interferencecriterion is transmitted through the material. Thus, in order to reflectone specific wavelength, the cholesteric material must have a pitchmatching that specific wavelength.

When the angle of incidence is greater than zero, i.e. at an obliqueangle of incidence, the effective pitch of the cholesteric material ischanged. By consequence, the colour reflected is altered accordingly bythe angle of incidence, as mentioned above. The wavelength λ_(max) forwhich maximum reflection occurs is given by:λ_(max) =n _(avg) p cos (α),where n_(avg) is the average refractive index of the material, p is thepitch of the cholesteric structure and α is the angle of incidence withrespect to the layer normal. It should be noted that, in the aboveequation, refraction at the layer surface between the cholestericmaterial and the ambient is neglected.

Clearly, as the angle of incidence increases, the wavelength of thelight reflected decreases. In other words, when a cholesteric filter isviewed under increasing angles, the wavelength of reflected lightdecreases. For display devices, this angle dependency is verydisturbing. For the red component of a multi-colour display, this isparticularly annoying, since the colour turns yellow or green. In thecontext, it should be noted that the human eye is most sensitive forgreen light. Therefore, the change from red towards green becomes moreapparent when a colour display based on cholesteric filters is viewedunder an oblique angle. Also, the reproduction of red colour isdeteriorated, leading to a loss of colour space.

One previously proposed solution to overcome the above-described problemis to introduce an absorbing filter, which absorbs the unwanted colours.However, such an approach to the problem has several limitations.Firstly, it reintroduces the absorbing colour filters. As mentionedabove, one advantage of using reflective filters (such as cholestericfilters) is the possibility of avoiding the expensive and complicatedabsorbing filters. Secondly, an absorbing filter will influence othercolours as well, thereby skewing the colour reproduction. A cholestericfilter layer comprising absorbing dyes which absorb unwanted colours isdisclosed in WO 00/33129.

Hence, in the art of reflective, cholesteric colour filters, there is aneed for improvements regarding the colour reproduction at obliqueviewing angles.

It is a general object of the present invention to provide a solution tothe above-described problems regarding the colour shift of cholestericfilters when viewed under oblique angles. This object is achieved by adevice and a method of the kind presented in the appended claims.

Hence, the present invention provides a reflective filter structurearranged to reproduce colour by reflection, said structure comprising afirst domain adapted to reflect, at normal angle of incidence, lighthaving a red colour, said filter structure being characterised byfurther comprising a second domain adapted to reflect, at normal angleof incidence, light having an infrared colour, wherein said seconddomain is further adapted to reflect, at a predetermined angle ofincidence greater than zero, light having a red colour.

In particular, it is an object of the present invention to increase theacceptable viewing angle of colour display devices based on reflectivefilters comprising a material having a cholesteric order.

Thus, it is an object of the present invention to provide a reflectivecolour filter based on cholesterically ordered liquid crystals, whichfilter exhibits an enhanced colour reproduction at oblique viewingangles without the need to incorporate absorbing elements. Nevertheless,it can be advantageous to combine the features of the present inventionwith absorbing elements or dyes in order to achieve particular, desiredeffects.

The present invention is based on the recognition that the addition ofan infrared domain to a conventional RGB pattern of a matrix display canbe used for the reproduction of red colour when the display is viewedunder oblique angles. The infrared domain is adapted to reflect, whenviewed under a normal angle of incidence, light within the infraredrange of the colour spectrum. Thus, under a normal (i.e. perpendicular)viewing angle, the infrared domain is invisible to the human eye.However, when viewed under an oblique angle, the wavelength of lightreflected from the infrared domain is shifted towards shorterwavelengths. Consequently, the colour reproduction from the infrareddomain can be arranged to fall within the red range of the colourspectrum when said domain is viewed under an oblique angle. By virtue ofthe infrared domain becoming visible red when viewed under obliqueangles, the defective yellow or green colour is compensated.

Although the invention is described by means of examples employingcholesteric liquid crystals, it is to be understood that other types ofreflective colour filters are possible within the scope of theinvention.

One advantage of the reflective filter structure according to thepresent invention is that the additional infrared domain can bemanufactured with existing technology. The colour reflected by acholesteric film is determined by the pitch of said film. Thus, aninfrared domain could easily be incorporated in a matrix display byproviding a domain of cholesteric material having a pitch that isdifferent from the pitch of the red, green and blue domains, forexample. The present invention gives important enhancement to reflectivecolour filters without introducing new and complicated processing steps.

Advantageously, the infrared domain is formed in a layer that is commonto all domains, i.e. the domains are formed side by side in a singlelayer. Alternatively, the infrared domain is formed in a second layer ontop of the layer comprising the red, green and blue domains. In thelatter case, the infrared domain is formed solely on top of the reddomain, leaving the green and blue domains uncovered. Similarly, theinfrared domain can also be formed in a second layer underneath the reddomain.

It should be noted that the terms “infrared”, “red”, “green” and “blue”domains refer to the colours reflected at a normal (i.e. perpendicular)viewing angle. However, the fact being appreciated that the reflectedcolour shifts to shorter wavelengths as the viewing angle increases.

In one aspect, the present invention provides a reflecting filterstructure of cholesteric material, wherein the reproduction of redcolour is augmented for oblique viewing angles. To this end, areflective filter structure comprises both a red domain and an infrareddomain. At zero angle of incidence (i.e. under normal viewing angle),the red domain reflects light of red colour, and the infrared domainreflects light of infrared colour. At an oblique angle of incidence, theinfrared domain reflects light of red colour, while the red domainreflects light of yellowish-green colour. As an example only, theinfrared domain can be adapted to reflect light within the red range ofthe colour spectrum when the filter is viewed at an angle of 45°. Thus,the reproduction of red colour is enhanced at an oblique viewing angleby the presence of the infrared domain.

In another aspect, the present invention provides a pixel structure forfull colour reproduction, the pixel structure comprising a blue domain,a green domain, a red domain and an infrared domain. Preferably, thearea of each domain is carefully selected in order to reproduce the bestpossible colour space. Different selections of domain sizes and domainlayout will be further described in the following detailed descriptionof some preferred embodiments of the present invention.

Manufacturing of patterned layers of cholesteric material is known inthe art. For example, WO 00/34808 discloses a method of manufacturing apatterned layer of a polymer material having a cholesteric order, inwhich the material is oriented is such a way that the axis of themolecular helix of the cholesterically ordered material extendstransversely to the layer.

Further features and objects of the present invention will beappreciated when the following detailed description of some preferredembodiments thereof is read and understood. In the detailed description,reference is made to the accompanying drawings, on which:

FIG. 1 schematically shows a prior art filter layer of cholestericallyordered material;

FIG. 2 schematically shows a reflective filter structure according tothe prior art;

FIG. 3 schematically shows a first embodiment of the present invention;

FIG. 4 schematically shows a second embodiment of the present invention;

FIG. 5 schematically shows a third embodiment of the present invention;

FIG. 6 schematically shows a fourth embodiment of the present invention;

FIG. 7 schematically shows a fifth embodiment of the present invention;and

FIG. 8 shows a plot of colour reproduction by reflective filtersaccording to both prior art and to the present invention.

By way of introduction, the improvements of reflective colour filtersachieved by the present invention will be described by way of anillustrative example. A layer of chiral nematic liquid crystals 10 (i.e.a cholesteric material) is schematically shown in FIG. 1.

Consider a reflective colour filter having a first domain of cholestericmaterial adapted to reflect, at normal angle of incidence, light havinga red colour. The wavelength of light perceived as red colour is in therange from 600 nm to 700 nm, typically around 650 nm. The pitch p of thecholesteric material required for reflecting light of the centrewavelength λ at normal incidence (α=0) in a material of averagerefractive index n_(avg) is given by p=λ/n_(avg). For non-zero angles ofincidence, the wavelength of reflected light is shifted towards shorterwavelengths (i.e. the wavelength of peak reflectivity is shifted towardsshorter wavelengths). If refraction at the interface between thecholesteric layer and the ambient (here assumed to be air having arefractive index of one) is accounted for, the centre wavelengthreflected at an angle of incidence α is given byλ(α)=λ₀ cos [sin⁻¹ (n _(avg) ⁻¹ sin (α))],where λ₀ is the wavelength reflected at zero angle of incidence andn_(avg) is the average refractive index of the cholesteric material.

If said first domain of cholesteric material is adapted to reflect lighthaving a wavelength of 650 nm at normal angle of incidence, thewavelength for which maximum reflection occurs at an oblique angle of ais given by the equation above. For example, at a viewing angle of 45°and an average refractive index n_(avg)=1.5, the reflected light will becentred around 573 nm (which is yellow).

Now, by juxtaposing, in the same layer, said first domain with a seconddomain of cholesteric material that is adapted to reflect, at normalangle of incidence, light in the infrared region of the spectrum, thereproduction of red colour at oblique angles is greatly enhanced. If thesecond domain is adapted to reflect light having a wavelength of about740 nm at normal angle of incidence, the reflected light at 45° angle ofincidence will be centred around 650 nm. Consequently, the loss ofreflection at 650 nm from the first domain is compensated by reflectionat 650 nm from the second domain. Hence, the perceived colour is amixture between red and yellow, rather than pure yellow. Preferably, thesizes of the first and the second domain, as well as the separationthere between, are small enough for the two domains not to be resolvedby the unaided human eye.

Alternatively, the infrared domain can be formed on top of the reddomain, in an additional layer. In this case, the infrared domain isprovided solely on top of the red domain, leaving the green and bluedomains uncovered. Similarly, the infrared domain can also be arrangedunderneath the red domain in a straightforward way.

It is envisioned that the present invention will have its primary fieldof use in colour display devices, such as RGB matrix displays. A priorart structure of an RGB display is illustrated in FIG. 2. Any pixel 20in the display comprises three sub-pixels: a red sub-pixel 21, a greensub-pixel 22 and a blue sub-pixel 23. Each sub-pixel 21, 22, 23 iscomprised of a domain of cholesteric material adapted to reflect, atnormal angle of incidence, a predetermined wavelength of light, i.e.red, green and blue light, respectively. In order to provide anupdateable pixel, the reflectivity of the cholesteric material can beswitched on and off, or adjusted, electrically.

However, as mentioned above, the angle of acceptance for a decent colourreproduction is only about ±20°.

According to the present invention, increased viewing angles areachieved by introducing into the pixel also an additional sub-pixel thatis adapted to reflect, at normal angle of incidence, light having aninfrared colour. Consequently, at oblique viewing angles, the reflectionfrom this additional sub-pixel will shift towards visible red.

In FIG. 3, a plan view of a first preferred embodiment of the presentinvention is shown. In this case, the conventional red domain is dividedinto one red domain 31 and one infrared domain 34. The shown embodimentis probably the most straightforward to manufacture, with a minimum ofalterations of the process. However, the reproduction of (the brightnessor reflection coefficient for) red colour is lower than the reproductionof green and blue, since the surface areas of the red 31 and theinfrared 34 domain, respectively, are smaller than the surface areas ofthe green 32 and blue 33 domains. In some cases, this might beacceptable or even desirable. In other cases, however, it may benecessary to take measures in order to avoid the difference in colourbrightness, such as the examples presented below.

In FIG. 4, a plan view of a second preferred embodiment of the presentinvention is shown. Here, the intensity of red, blue and green light isbalanced by each of the infrared 44, the red 41, the green 42 and theblue 43 domains having substantially equal surface areas. As the viewingangle increases, and as the reflection of red light from the red domain41 decreases, the reflection of visible red from the infrared domain 44increases accordingly. It is to be understood that the area of eachdomain 41, 42, 43, 44 can be designed so that the desired colourbrightness is achieved.

FIG. 5 schematically shows a plan view a third preferred embodiment ofthe present invention. In this embodiment, the red R and the infrared IRdomains are interspersed in a checkerboard structure. The checkerboardstructure enhances the mixture of the light reflected from the reddomain and the light reflected from the infrared domain.

In FIG. 6, a plan view of a fourth embodiment of the present inventionis shown. This embodiment is similar to that shown in FIG. 5. However,in order to further enhance the reproduction of red colour, thecheckerboard structure of the red R and infrared IR reflective domainsis larger than in the previous embodiment. By choosing the surface areasof each domain (infrared, red, green and blue) appropriately, thereproduction of red colour can be balanced to the reproduction of greenand blue colour.

FIG. 7 schematically shows a side view of an embodiment of the presentinvention, in which an infrared IR domain is arranged in a second layeron top of the layer comprising the red R, green G and blue B domains.The infrared IR domain is provided only over the red R domain,effectively leaving the green and blue domains uncovered. For practicalreasons, also the green and blue domains may be covered, but with anon-active NA domain that is essentially non-reflecting. It is to beunderstood that the effect of the infrared IR domain on the colourreproduction of the green G and blue B domains should be avoided to thegreatest possible extent.

Although the infrared domain is shown to be provided on top of the reddomain in FIG. 7, it is also possible to have the infrared domainunderneath the red domain.

The improved colour reproduction at increasing viewing angles achievedby the present invention is illustrated in FIG. 8. The figure shows, ina colour triangle plot, the colour reproduction of a single domain redfilter (squares), and a dual-domain, red and infrared filter(triangles). In the colour triangle, the right corner corresponds to redcolour, the upper corner to green colour and the lower/left corner toblue colour. The red domain had a 100 nm wide reflection band centred at650 nm, and the infrared domain had a 100 nm wide reflection bandcentred at 750 nm. In the dual-domain filter, the surface area of thered domain was equal to the surface area of the infrared domain. Thecolour of the reflected light is plotted for viewing angles in air(outside of the colour filter) ranging from zero angle of incidence to60° angle of incidence, in steps of 5°. The figure clearly illustratesthat the reproduction of red colour is greatly enhanced when adual-domain filter according to the present invention is utilised.

Hence, by including an infrared domain according to the presentinvention, i.e. a reflective filter layer domain reflecting light withinthe infrared range of the spectrum when viewed from zero angle ofincidence and reflecting visible red when viewed from an oblique angleof incidence, in a reflective colour filter based on chirally nematic(cholesteric) liquid crystals, the colour reproduction at oblique anglesis considerably enhanced.

1. A reflective filter structure arranged to reproduce colour byreflection, said structure comprising a first domain adapted to reflect,at normal angle of incidence, light having a red colour, said filterstructure being characterised by further comprising a second domainadapted to reflect, at normal angle of incidence, light having aninfrared colour; and to reflect, at a predetermined angle of incidencegreater than zero, light having a red colour.
 2. A reflective filterstructure as claimed in claim 1, further comprising a third domainadapted to reflect, at normal angle of incidence, light having a greencolour.
 3. A reflective filter structure as claimed in claim 1 or 2,further comprising a fourth domain adapted to reflect, at normal angleof incidence, light having a blue colour.
 4. A reflective filterstructure as claimed in any one of the preceding claims, wherein atleast one of the reflective domains comprises cholesteric liquidcrystals.
 5. A reflective filter structure as claimed in claim 1,wherein the first domain is interspersed with the second domain in acheckerboard pattern.
 6. A reflective filter structure as claimed inclaim 1, wherein the reflective surface area of the first domain issubstantially equal to the reflective surface area of the second domain.7. A reflective filter structure as claimed in claim 3, wherein thereflective surface area of each domain is essentially equal.
 8. Areflective filter structure as claimed in any one of the precedingclaims, wherein each domain is formed in a single layer.
 9. A reflectivefilter structure as claimed in any one of the claims 1 to 7, wherein thesecond domain is formed in a second layer provided on either side of alayer comprising the first domain.
 10. A liquid crystal colour displaydevice of the reflective type, in which each pixel comprises a domain ofcholesteric material adapted to reflect, at normal angle of incidence,light of red colour, a domain of cholesteric material adapted toreflect, at normal angle of incidence, light of green colour, a domainof cholesteric material adapted to reflect, at normal angle ofincidence, light of blue colour, the display device being characterisedby further comprising an additional domain of cholesteric materialadapted to reflect, at normal angle of incidence, light of infraredcolour; and to reflect, at a predetermined angle of incidence greaterthan zero, light having a red colour.